Various signal processing methods are known to improve quality of speech signals, e.g., by suppressing signals other than speech signals like acoustic echoes during a voice call. It is also known that these methods may attain higher performance if speech processing thereof is divided into a former stage and a latter stage.
A method of echo cancellation, called a first method, is disclosed in Japanese Patent Publication (Toroku), No. 3420705, which is divided into a former stage of processing and a latter stage of processing. According to the first method, residual signal is produced by canceling echo in the former stage, and output signal on send path is produced by suppressing echo included in the residual signal in the latter stage. A short-time spectral amplitude is estimated in the latter stage using the output signal on send path and the echoes are suppressed based on the above estimate.
According to the first method described above, in a case where linear echo cancellation (adaptive filtering) in the former stage and nonlinear echo cancellation in the latter stage are combined, a filter coefficient of the adaptive filter is diverted on an assumption that acoustic coupling level (echo path loss) of an echo path does not vary much upon a change of the echo path during a double talk. Therefore the echo may not be canceled adequately in a case where the adaptive filter shows poor estimating capability e.g., upon a change of the echo path during a double talk.
Another method of echo cancellation, called a second method, is disclosed in Japanese Patent Publication (Kokai), No. 2004-56453. In a former stage of processing of the second method, an adaptive filter cancels an echo component included in a sent speech input from a microphone. In a latter stage of processing of the second method, an echo or noise is suppressed based on an echo replica signal produced in the former stage.
According to the second method described above, capability to estimate an echo path depends upon capability of an adaptive filter of the former stage to estimate the echo path, as the second method always uses the echo replica signal. It is assumed that the adaptive filter can estimate an echo precisely in the former stage. If there is a steep change of the echo path or a change of echo path loss during a double talk, a residual echo is likely to remain in the output signal on send path, i.e., the output of the latter stage.
There may be a variation of echo path loss during double talk or a steep change of an echo path. Besides, performance of echo cancellation in a former stage of processing, whether linear processing or non-linear, may be inadequate. In such a case, according to the first and the second methods, accuracy of an echo level estimate or a noise level estimate in echo cancellation, noise suppression or a combination of them in a latter stage of processing may deteriorate due to a residual echo in an output signal of the former stage.
Due to the above deterioration, there may be a problem that performance of echo cancellation in the latter stage is inadequate, and so is performance of noise suppression in the latter stage. The above problem often occurs with echo suppression on a voice switch in the latter stage.